Conventional systems for treating domestic waste water, including domestic septic systems, have concentrated on oxidising organic carbon, and on completing the nitrification of ammonia to nitrate.
It has largely been left to naturally-occurring processes to remove phosphorus in the waste-water. However, the problem of contamination of lakes etc by phosphorus is troublesome in some cases to the extent that further habitation cannot be allowed around the lake unless steps are taken to reduce the phosphorus contamination.
It is recognised that there is a need for a treatment system for removing phosphorus from waste water, that is effective, does not put other pollutants into the water, is passive (in the sense of needing little input of energy, replenishment materials, and attentive and skilled service). It is recognised that, in order to be acceptable from the regulatory standpoint, the materials the system uses have to be very cheap.
It is an aim of the invention to provide a barrier of a very inexpensive treatment material, in the flow-path of the contaminated water, the barrier being permeable enough to allow the water to pass therethrough, and the treatment material being such as will remove the phosphorus and other contaminants.
It is known to inject liquids such as FeCl.sub.3 or Al.sub.2 (SO.sub.4).sub.3 into municipal sewage; this causes precipitation of, for instance, FePO.sub.4, which collects as sludge. However, as a general rule, treatment by adding a liquid is disadvantageous from the service and maintenance side, in that the liquid cannot be added just once and then left, but must be added regularly. Thus, an add-a-liquid treatment system is contra-indicated for the domestic situation. Domestically, a passive system is desirable, which means, at least insofar as the treatment system is to be simple and inexpensive, that the treatment material must be a solid. Also, in a domestic system, by contrast with a municipal system, a treatment mechanism that causes the formation of sludge is contra-indicated, because sludge would clog the tile-bed or other soakaway facility. In the domestic situation low cost, and passiveness, are critical.